1. Field of the Invention
The invention relates to a device and a method for the selection of a traffic parameter such as the bit rate for HF (high frequency) transmissions using a modem with equalizer.
The invention can be applied especially to all HF digital communications using a serial modem with equalizer whatever its waveform and whatever the traffic carried, for example data, digitized voice transmission, traffic in FEC (Forward Error Correction) mode and ARQ (Automatic Request Query) mode, and whatever the type of link, for example links in the half-duplex or full-duplex mode, unicast links or broadcast links etc.
Hereinafter in the description, the expression “traffic parameter” designates one or more parameters from the following list: the frequency, the useful data transmission bit rate, the waveform, the interleaver and/or the power.
Similarly, the expression “propagation parameters” covers for example the SNR (Signal-to-Noise Ratio), the nature and the level of the interference, the Doppler spread, the temporal spread, the dynamic range of the multiple paths, the shift and the Doppler ramp.
In the field of HF digital transmission, propagation by ionospheric reflection uses a non-stationary medium that is extremely fluctuating in time and in space with short-term and long-term variations in propagation parameters, such as those mentioned here above.
The advent of the serial modem (or single-tone modem) with equalizer has made it possible to augment the user bit rate in digital communications. The equalizer can be used to overcome fast variations in the channel and especially the fading of the envelope of the received signal.
The bandwidth of the channel is also variable and the ALE (Automatic Link Establishment) protocols enable the automatic selection of an unoccupied and active frequency in the range between the Lowest Usable Frequency (or LUF) and the Maximum Usable Frequency (or MUF) in order to provide quality of service or QoS at the receiver point, for the data traffic that follows the ALE phase.
Since the transmission medium is variable, the HF transmission mode should be capable of adapting. This implies the selection, at the beginning of the link and during the link, of the traffic parameters best suited to the propagation characteristics of the channel, the noise or the interference (on the receiver side) depending for example on the quality of service required by the user. The selected traffic parameters are especially the frequency, the bit rate, the waveform, the interleaver and/or the power.
2. Description of the Prior Art
To this end, the prior art describes various adaptive procedures that may be classified under several categories depending on the parameter of the traffic that is adapted and the nature of the method, namely the prediction or estimation of the propagation channel.
FIG. 1 gives a diagrammatic view of a prior art method that implements a waveform.
The signal S0 received by the modem after transmission through the ionosphere is processed in a channel estimator 1 whose function especially is to carry out the extraction, by using an algorithm of varying complexity (for example of the MUSIC or Multiple Signal Classification type), of the estimated propagation parameters 2 during the reception of a polling waveform. A management software program 4 carries out what is known as a “matching process” type processing by crossing these estimated parameters with performance curves obtained for different types of modems and stored in one or more databases 3. Then, depending for example on a required quality of service QoS, 5, it determines a set 6 of traffic parameters to be implemented. Thus, the method determines the optimum bit rate value for transmitting data.
The term “quality of service” or QoS designates for example the bit error rate (BER), the end-to-end time of the transmission line, etc. The set of traffic parameters comprises for example the following parameters: the waveform used for data transmission, the user bit rate, the length of the interleaver or any other traffic parameter.
However, such a system is complicated and requires CPU power and memory size, the performance curves being multi-dimensional. Thus, for a given transmission bit rate Di, the bit error rate BER is expressed as a function of the signal-to-noise ratio, the value of the Doppler spread and the temporal spread of each path, the amplitude and the number of these paths etc. Furthermore, a channel estimation algorithm lacking high resolution would give rise to some uncertainty in the bit error rate in a ratio of 1 to 100. This indirect method in which the estimation of the propagation parameters is separated from the performance tables therefore proves to be complex and of low efficiency.
In another simpler technique, the propagation parameters are reduced simply to the signal-to-noise ratio, or SNR. This technique however proves to be inoperative in the presence of multiple paths. Indeed, the signal-to-noise ratio does not represent quality of service for an ionospheric medium. There is a high variation in the quality of service QoS for a fixed SNR when the Doppler spread and/or the temporal spread is made to vary.